This invention relates to monitoring diagnosis devices for electrical appliances, and more particularly to monitoring diagnosis devices for monitoring the occurrences of abnormality and diagnosing, identifying and inferring the causes of abnormality of electrical appliances, such as gas insulated electric devices, oil insulated electrical appliances, air insulated electrical appliances, vacuum insulated electrical appliances, and solid insulated electrical appliances, for interrupters, switchgears, disconnectors, voltage transformers, current transformers, buses, insulated meters, generators or dynamos, and electrical rotating machines.
FIG. 12 is a diagrammatic view showing the structure of a conventional monitoring diagnosis device, which is disclosed in: T. Yamagiwa et al., "DEVELOPMENT OF PREVENTIVE MAINTENANCE SYSTEM FOR HIGHLY RELIABLE GAS INSULATED SWITCHGEAR", 90 WH 160-2 PWRD, 1990 IEEE.
In FIG. 12, an electrical appliance (in this case a gas insulated switchgear) includes a spacer 1 and a tank 2. A central conductor 3 accommodated within the tank 2 is connected to a power transmission line 4. A foreign object 5 may be attached to the central conductor 3 to cause partial discharges 6 within the tank 2. An acceleration sensor 7 and an ultrasonic sensor 39 mounted on the tank 2 detect the low and the high frequency vibrations. A divider circuit 40 calculates the ratio C of the output A of the acceleration sensor 7 and the output B of the ultrasonic sensor 39. A judgment circuit 41 judges whether or not the ratio C is greater than a predetermined threshold level Th. Thus, a controller 42 including the divider circuit 40 and the judgment circuit 41 monitors the occurrences of abnormality and diagnoses the causes thereof in accordance with a predetermined algorithm, outputting the monitoring signal D.
Next, the operation of the monitoring diagnosis device of FIG. 12 is described. The divider circuit 40 calculates the ratio C of the outputs A and B of the acceleration sensor 7 and the ultrasonic sensor 39. On receiving the ratio C, the judgment circuit 41 judges that an abnormality has occurred in the gas insulated switchgear when the ratio is greater than the threshold Th. When the ratio is less than the threshold Th, the judgment circuit 41 judges that the outputs of the acceleration sensor 7 and the ultrasonic sensor 39 are noises and hence that no abnormality is present. The level of the threshold Th is set in accordance with experimental results or experience.
Thus, the conventional monitoring diagnosis device has the following disadvantage. The occurrences of abnormality are determined in accordance with the algorithm by which the ratio C of the outputs of two sensors is compared with a predetermined threshold Th and an occurrence of abnormality is determined when the ratio is greater than the threshold Th. The threshold Th must therefore be determined by means of experiments or experience. This determination is time consuming. The determination of the threshold Th must be renewed when the type of the gas insulated switchgear changes, or when the installation site and hence the background noise environment change. Further, it is necessary to update the level of the threshold Th in accordance with the aging of the device. Furthermore, the determination algorithm requires two sensors. When only one sensor is available, the algorithm cannot be used. On the other hand, even if three or more sensors are installed, the information supplied by the additional sensors cannot be utilized in the diagnosis. Thus applicability of the monitoring diagnosis device is limited. Further, the conventional monitoring diagnosis device provides only the information on whether the gas insulated electric device is in an abnormal or normal state: the causes of abnormality cannot be identified.